Phantoms are routinely used for performance testing and quality assurance of nuclear imaging systems. While simple phantoms are used for basic performance tests such as resolution and uniformity, complex phantoms are used to simulate more realistic patient imaging situations and diagnostic tasks such as lesion detection. However, existing designs for such phantoms have limited accuracy, reproducibility, and ease of use because multiple chambers have to be filled with the exact activity concentrations desired. These limitations have restricted the widespread use of complex phantoms for quality assurance. In Phase I, we propose a new design approach for complex phantoms that overcomes the above limitations. We propose to develop a prototype phantom for lesion conspicuity studies and to validate its accuracy through experimental tests on SPECT and PET scanners. Our design will achieve excellent reproducibility and accuracy while greatly improving ease of use. In Phase II, we will develop advanced phantom designs that accurately simulate patient imaging and that are suitable for performance testing of attenuation correction and scatter correction. This research will lead to commercial products that will be very valuable for routine quality assurance, for clinically relevant performance comparisons between scanners, and for research studies. PROPOSED COMMERCIAL APPLICATIONS: The research will lead to a commercial phantom for nuclear imaging that is more accurate, reproducible, and convenient to use than existing phantoms. The phantom will be used for acceptance testing, quality assurance, performance comparisons, and research for SPECT and PET scanners.